Over the past 20 years, the ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has established itself as an invaluable tool for studying the morphology and electronic structure of surfaces at the atomic scale. During this time, the minimum lateral feature size of silicon-based microelectronics has reached 90 nm and will soon make a transition to 65 nm technology. In this context, the STM has been used to develop a new understanding of silicon surfaces and to study the adsorption and desorption of individual atoms and molecules from silicon surfaces. For example, the study of STM-induced hydrogen and deuterium desorption from Si(100) surfaces has directly impacted the performance and reliability of microelectronics technology. Using a monolayer of hydrogen as a chemical resist, the STM has also been used to pattern and study molecular adsorption on the Si(100) surface. Furthermore, through feedback-controlled lithography (FCL), individual molecules have been patterned on silicon surfaces with atomic resolution. The study of individual molecules adsorbed onto the silicon surface, both isolated and adsorbed at controlled distances from one another, may enable the development of complementary molecular-scale electronic technologies that can be integrated with conventional microelectronic circuits.
When studying the adsorption of individual molecules on surfaces, a cryogenic variable temperature UHV STM offers a wider and more controlled experimental phase space than a room temperature UHV STM. In particular, at cryogenic temperatures, surface diffusion of physisorbed species may be controlled or eliminated, and thermal drift can be significantly attenuated. Much previous work of UHV STM manipulation and scanning tunneling spectroscopy (STS) of individual atoms and molecules has focused on metal surfaces at cryogenic temperatures. In these cases, the cryogenic UHV STM was used to manipulate individual physisorbed species that would be highly mobile at elevated temperatures, thus enabling the STM to make and break individual chemical bonds in a more controlled way. Furthermore, by reducing thermal broadening of the probing electronic energy distribution, spectroscopic resolution is improved at cryogenic temperatures. Through STS and inelastic electron tunneling spectroscopy, the cryogenic UHV STM has been used as a tool for the chemical identification of individual adsorbed molecular species and for obtaining the intra-molecular electronic structure of both randomly adsorbed molecules and patterned molecular arrays.
A variety of cryogenic variable temperature UHV STM designs have been presented in the literature. A common design problem centers on a trade-off between adequate coupling to the cryogen to maintain a stable low temperature while being sufficiently decoupled from any source of environmental vibrations. In many designs, the STM is clamped directly to a cryostat, and the cryostat and STM are vibrationally isolated. In other active cooling designs, a flexible thermal link exists between the STM and cryostat. In passive cooling schemes, the STM stage is located inside an isothermal enclosure, is then clamped to the cryostat for cooling, and is subsequently unclamped to perform STM.